Sensors and sensor systems implemented in integrated circuits (ICs) often have analog output interfaces for communicating sensor signals to other system components, such as an electronic control unit (ECU) or other device in, e.g., automotive applications. Analog output interfaces often comprise digital-to-analog converters (DAC) and output drivers such that the output voltage from the driver is communicated external to the device via a conductor. In the event of a line interruption in the external conductor, for example, a break in the conductor itself, the output could be floating or other undesirable or even damaging errors can occur. Thus, on-board diagnostic (OBD) components can be implemented in the sensor systems to detect external line interruptions like this.
In one configuration, two OBD circuits are provided within an IC. The first OBD circuit is coupled between the output of the analog output interface and the terminal for supply potential VDD, and the second OBD circuit is coupled between the output and the terminal for a negative supply potential or reference-ground potential GND. The OBD circuit essentially is a switch, comprising, e.g., a depletion transistor, and is normally open when the IC is operating correctly. In the event of an external fault, such as a line interruption that causes the IC to lose power, the OBD circuit closes and forms a short circuit such that the output signal is pulled towards VDD or GND to indicate an error.
Conventional sensors commonly provide diagnostics by defining OBD diagnostic output values or clamping ranges such that the operating range of the analog output is reduced, for example, to 10% to 90% VDD. This disadvantageously limits the external output signal level. Additionally, certain safety requirements, for example, Automotive Safety Integrity Level (ASIL) standards, provide for diagnostics of internal functionalities. In order to fulfill these requirements, it is typical to define expanded diagnostic output values or clamping ranges so that the operating range of the analog output is further reduced, for example, 20% to 80% VDD, resulting in a more reduced output signal level which can be a significant disadvantage. Another disadvantage is that if a failure occurs internal to the analog output interface itself, possibly due to failure of the DAC or output driver, the diagnostic output value or clamping range is no longer transmitted. This also can significantly reduce the diagnostic coverage since the output stage often needs a significant chip area in order to provide safety diagnostics for all or at least critical ones of the IC components.
In many applications, there is a general need for a sensor that is able to detect errors internal, as well as external, to the sensor. A drawback of some conventional approaches, then, is the limiting of the remaining signal level. A further drawback of conventional approaches is the dependence on the DAC and output driver such that failure in the DAC or output driver results in the non-transmission of the clamping range. A further disadvantage in conventional sensors that provide internal detection error is the number of additional components needed to provide diagnostic capabilities, thus increasing the size, cost, and complexity of the circuit.